1. Field of the Invention:
The invention relates to luminescent materials and somewhat more particularly to humidify-stabilized crystals of the activated rare-earth oxybromide type.
2. Prior Art:
Luminescent materials of the activated rare-earth oxybromide type are known, for example see German Offenlegungsschrift (hereinafter DT-OS) No. 28 15 679. Such oxybromide luminescent materials are typically very stable under dry storage conditions. However, they decompose very rapidly under humid conditions. Even when, for example, rare-earth activated lanthanum oxybromide materials are compressed into tablets, they disintegrate in a few days at ambient humidity typically present at room temperatures.
Heretofore, attempts have been made to stabilize luminescent materials against humidity in various ways. One such method is described in the above-referenced DT-OS. In this process, the sensitivity of the lanthanum oxybromide to moisture is reduced by converting the surface of the LaOBr-crystals into oxyfluoride or oxychloride. Crystals are thus obtained which have a chloride or fluoride outer coating which protects against decomposition that may be caused by atmospheric humidity. However, a disadvantage of this type of stabilization is that the halide coating causes a perceptible loss of luminescence, which can amount up to 10% to 30%, relative to identical uncoated or non-stabilized luminescent material. Further, due to a change of halides, traps or absorbance centers in the luminescent materials are produced which render such materials more sluggish in light-decay, ie., the luminescence persistence or afterglow is increased by up to 100%. Yet further, the exchange of halides which occurs at the edge or boundaries of the so-stabilized crystals can only be reproduced with difficulty, particularly because such halide coatings are achieved with a treatment by gaseous HCl of HF.